There are methods and devices in which, within the scope of a first diagnostic process, a first temperature signal of a first temperature sensor in the internal combustion engine ascertained at a first point in time after switching off the internal combustion engine is compared to a second temperature signal of a second temperature sensor in the internal combustion engine ascertained at the first point in time. An error is identified when in the comparison it is ascertained that the absolute value of the difference between the first temperature signal and the second temperature signal exceeds a predefined first threshold value for the first point in time, the predefined first threshold value being predefined to be smaller the longer the time since the internal combustion engine has been switched off.
An identified error is cleared in a subsequent second diagnostic process; i.e., a corresponding error memory is reset when, within the scope of the subsequent second diagnostic process, for the difference between the first temperature signal and the second temperature signal for a second point in time it is ascertained that the absolute value of the second difference does not exceed a second threshold value predefined for the second point in time.
The problem is the following: the diagnostic accuracy is a function of the time that has elapsed, at the first point in time or at the second point in time, since the internal combustion engine was switched off. The longer the time period since engine switch-off, the smaller the difference between the temperature signals of the two temperature sensors must be in the error-free case. Therefore, the threshold value for comparison with the difference between the two temperature signals is predefined to be smaller the longer the time period since engine switch-off, i.e., the time since the internal combustion engine has been switched off. A minor, incorrect difference between the temperature signals of the two temperature sensors can be identified only if the diagnosis is carried out after an appropriate period of time has elapsed since switching off the engine.
However, if the period of time since switching off the engine is selected to be much shorter for the subsequent second diagnostic process under the same ambient conditions, for example because the internal combustion engine is restarted much earlier after being switched off, the diagnosis is carried out for a much earlier point in time than for the first diagnostic process, so that, as a result of the threshold value for the second diagnostic process which is then larger, the minor, incorrect difference between the two temperature signals is no longer identified as an error; on the contrary, the error previously placed in the error memory in the first diagnostic process is erased, i.e., cleared. However, such an error clearing is not justified.